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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Peter Zijlstra' <peterz@infradead.org>
CC:     'Steven Rostedt' <rostedt@goodmis.org>,
        'Vincent Guittot' <vincent.guittot@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        "Ingo Molnar" <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        "Dietmar Eggemann" <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>,
        "Mel Gorman" <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: RE: sched/fair: scheduler not running high priority process on idle
 cpu
Thread-Topic: sched/fair: scheduler not running high priority process on idle
 cpu
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Date:   Wed, 15 Jan 2020 15:09:56 +0000
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From: Peter Zijlstra <peterz@infradead.org>
> Sent: 15 January 2020 14:57
> On Wed, Jan 15, 2020 at 12:44:19PM +0000, David Laight wrote:
> 
> > Code that runs with a spin-lock held (or otherwise disables preemption)
> > for significant periods probably ought to be detected and warned.
> > I'm not sure of a suitable limit, 100us is probably excessive on x86.
> 
> Problem is, without CONFIG_PREEMPT_COUNT (basically only
> PREEMPT/PREEMPT_RT) we can't even tell.
> 
> And I think we tried adding warnings to things like softirq, but then we
> get into arguments with the pure performance people on how allowing it
> longer will make their benchmarks go faster.

The interval would have to be a sysctl - like the one for sleeping uninterruptibly.
(Although that one is a pain for some kernel threads. I'd like to be able to
mark some uninterruptible sleeps as 'long term' and also not affecting the load
average.)

I remember (a long time ago) adding code to an ethernet driver to limit it
to 90% of the bandwidth to allow other systems to transmit (10M HDX).
Someone said ' we can't do that, people expect 100%', a week later he
asked me how to enable it because the AMD Lance could never transmit
if receiving back to back packets  (eg in promiscuous mode).
Benchmarks are a PITA....

	David

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